Thin SRAM Cell Having Vertical Transistors

ABSTRACT

A memory device includes six field effect transistors (FETs) formed with semiconductor nanowires arranged on a substrate in an orientation substantially perpendicular to the substrate. The semiconductor nanowires have bottom contacts, gate contacts separated in a direction perpendicular to the substrate from the bottom contacts, and top contacts separated in a direction perpendicular to the substrate from the gate contacts. The necessary connections are made among the bottom, gate, and top contacts to form the memory device using first, second, and third metallization layers, the first metallization layer being separated in a direction perpendicular to the substrate from the top contacts, the second metallization layer being separated in a direction perpendicular to the substrate from the first metallization layer, and the third metallization layer being separated in a direction perpendicular to the substrate from the second metallization layer. Vias connect the various contacts to the overlying metallization layers as necessary. A method for forming the memory device is also outlined.

TECHNICAL FIELD

This disclosure relates to semiconductor nanowire field effecttransistors and to memory devices formed therefrom.

BACKGROUND

The six transistor (6T) static random access memory (SRAM) cell is theprimary memory used in microprocessor circuits. As is well known tothose of ordinary skill in the art, continued efforts are being made todesign integrated circuit chips having the greatest possible number ofindividual 6T SRAM cells, in order to provide the integrated circuitchip with as much memory as possible within the available area thereon.To achieve this goal, integrated circuit designers have been developinglayouts for the transistors making up the cells to reduce the arearequired for each. The present invention provides one approach towardmeeting this challenge.

SUMMARY

In one aspect of the present invention, a memory device comprises afirst plurality of semiconductor nanowires for a first, a second, and athird field effect transistor (FET) and a second plurality ofsemiconductor nanowires for a fourth, a fifth, and a sixth FET. Thefirst and second pluralities of semiconductor nanowires are arranged ona substrate with the semiconductor nanowires being orientedsubstantially perpendicular to the substrate.

The memory device further comprises a plurality of bottom contacts forthe semiconductor nanowires on the substrate. Each of the semiconductornanowires has a bottom contact. The one or more of the semiconductornanowires of said first plurality for the third FET share a bottomcontact with the one or more of the semiconductor nanowires of thesecond plurality for the fourth FET.

The memory device also comprises a plurality of gate contacts separatedin a direction perpendicular to the substrate from the plurality ofbottom contacts. Each of the semiconductor nanowires of the first andsecond pluralities has one of the gate contacts. Two or moresemiconductor nanowires of the first plurality for the second and thirdFETs share a gate contact with one another, and two or moresemiconductor nanowires of the second plurality for the fourth and fifthFETs share a gate contact with one another.

The memory device additionally comprises a first top contact for thefirst plurality of semiconductor nanowires, and a second top contact forthe second plurality of semiconductor nanowires. The first and secondtop contacts are separated in a direction perpendicular to the substratefrom said plurality of gate contacts.

As a consequence, the third and fourth FETs share a bottom contact; thesecond and third FETs share a gate contact; the fourth and fifth FETsshare a gate contact; the first, second, and third FETs share a topcontact; and the fourth, fifth, and sixth FETs share a top contact.

Finally, the gate contact of the second and third FETs is connected tothe top contact of the fourth, fifth, and sixth FETs; the gate contactof the fourth and fifth FETs is connected to the top contact of thefirst, second, and third FETs; the gate contacts of the first and sixthFETs are connected to word line (WL) connections; the bottom contacts ofthe second and fifth FETs are connected to voltage source (Vdd)connections; the bottom contact of the first FET is connected to asecond bit line (BLB) connection; the bottom contact of the sixth FET isconnected to a first bit line (BL) connection; and the shared bottomcontact of the third and fourth FETs is connected to a ground (Vss)connection.

In another aspect of the present invention, a method for forming amemory device comprises forming a first plurality of semiconductornanowires for a first, a second, and a third field effect transistor(FET) and a second plurality of semiconductor nanowires for a fourth, afifth, and a sixth FET on a substrate. The first and second pluralitiesof semiconductor nanowires are arranged with the semiconductor nanowiresbeing oriented substantially perpendicular to the substrate.

The method for forming a memory device also comprises forming aplurality of bottom contacts for the semiconductor nanowires on thesubstrate. Each of the semiconductor nanowires has a bottom contact. Theone or more of the semiconductor nanowires of the first plurality forthe third FET share a bottom contact with the one or more of thesemiconductor nanowires of the second plurality for the fourth FET.

The method for forming a memory device further comprises forming aplurality of gate contacts separated in a direction perpendicular to thesubstrate from the plurality of bottom contacts. Each of thesemiconductor nanowires of the first and second pluralities has one ofthe gate contacts. Two or more semiconductor nanowires of the firstplurality for the second and third FETs share a gate contact with oneanother, and two or more semiconductor nanowires of the second pluralityfor the fourth and fifth FETs share a gate contact with one another.

The method for forming a memory device additionally comprises forming afirst top contact for the first plurality of semiconductor nanowires anda second top contact for the second plurality of semiconductornanowires. The first and second top contacts are separated in adirection perpendicular to the substrate from the plurality of gatecontacts.

As a consequence, the third and fourth FETs share a bottom contact; thesecond and third FETs share a gate contact; the fourth and fifth FETsshare a gate contact; the first, second, and third FETs share a topcontact; and the fourth, fifth, and sixth FETs share a top contact.

Finally, the method for forming a memory device further comprisesconnecting the gate contact of the second and third FETs to the topcontact of the fourth, fifth, and sixth FETs; connecting the gatecontact of the fourth and fifth FETs to the top contact of the first,second, and third FETs; connecting the gate contacts of the first andsixth FETs to word line (WL) connections; connecting the bottom contactsof the second and fifth FETs to voltage source (Vdd) connections;connecting the bottom contact of the first FET to a second bit line(BLB) connection; connecting the bottom contact of the sixth FET to afirst bit line (BL) connection; and connecting the shared bottom contactof the third and fourth FETs to a ground (Vss) connection.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of these teachings are made more evidentin the following detailed description, when read in conjunction with theattached drawing figures.

FIG. 1 is a circuit diagram of a static random access memory (SRAM).

FIG. 2 is a plan view taken in an axial direction of nanowires useful inthe practice of the present invention.

FIG. 3 illustrates the bottom contacts for the nanowires of FIG. 1.

FIG. 4 illustrates the gate patterning for the nanowires of FIG. 1.

FIG. 5 illustrates the top contacts for the nanowires of FIG. 1.

FIG. 6 shows the bottom contacts, the gate patterning, and the topcontacts for nanowires in a first alternate embodiment of the invention.

FIG. 7 shows the bottom contacts, the gate patterning, and the topcontacts for nanowires in a second alternate embodiment of theinvention.

FIG. 8 is a plan view of the vias for connecting the bottom contacts,the gate contacts, and the top contacts for the embodiment of FIG. 5 tothe first metallization layer.

FIG. 9 is a plan view of the first metallization layer.

FIG. 10 is a plan view of the second metallization layer.

FIG. 11 is a plan view of the third metallization layer.

FIG. 12 shows the location of a SRAM cell of the present inventionrelative to its neighbors of the same type in the vertical andhorizontal directions.

DETAILED DESCRIPTION

An integrated circuit may include a number of different types of fieldeffect transistor (FET) devices that may be formed from nanowire channelFETs. A nanowire channel FET includes a silicon nanowire that connectsto a source region and a drain region and a gate that fully (orpartially) surrounds the nanowire or nanowires. The channel forms at thesurface of the nanowires under the gate, or in the bulk of the nanowiresfor nanowires with diameter smaller than about 5 nm. When the gate fullysurrounds the nanowire, the device is referred to as a gate-all-around(GAA) FET. When the gate partially surrounds the nanowire, as in thecase where the nanowire is attached to an insulator, the device isreferred to as an omega-gate FET.

Nanowire FETs may be fabricated to form, for example, nFET and pFETdevices. The nFET and pFET devices may be connected to form a variety ofintegrated circuit devices such as, for example, inverters and staticrandom access memory (SRAM) cells. It is generally desirable in circuitdevices for FETs to be matched by having, for example, similar thresholdvoltages and drive current.

In general, nano wire FET devices that are formed on a wafer may includeany number of nanowires. The fabrication process may include, forexample, forming a silicon nanowire on a buried oxide (BOX) substrateusing an isotropic etching process. The etching process results in anelliptically (including cylindrically) shaped nanowire that may besuspended above the substrate or may be partially disposed on thesubstrate. A metallic or polysilicon gate structure is formed on thenanowire. Source and drain regions are formed adjacent to the gatestructure, and contacts may be formed to connect the source, drain, andgate structure to other devices.

The fabrication process may result in particular nanowires havingdifferent properties such as, for example, the diameter of one nanowireon a wafer may be different from the diameter of another nanowire due tothe location of the particular nanowire on the wafer. Though thediameters of two different nanowires may vary on a wafer, the diameterof each particular nanowire is typically constant, and within a desiredtolerance.

Integrated circuit components, such as, for example, SRAM cells andinverters, include a number of pFET and nFET devices disposed onnanowires that are arranged on a wafer. Since the properties of thenanowires (e.g., nanowire diameters) affect the operation of thedevices, it is desirable to arrange the devices such that the effects ofthe differences in the nanowire properties are reduced.

FIG. 1 is a circuit diagram of a static random access memory (SRAM). TheSRAM includes an nFET device (M₆) 101 connected to a first bit line node(BL) 102, a first output node (Q) 104, and a word line node (WL) 106. AnnFET device (M₃) 103 is connected to the Q node 104, a ground node (Vss)108, and a second output node (Q′) 110. A pFET device (M₄) 105 isconnected to the Q node 104, the Q′ node 110, and a voltage source node(Vdd) 112. A pFET device (M₂) 107 is connected to the Vdd node 114, theQ node 104, and the Q′ node 110. An nFET device (M₁) 109 is connected tothe Vss node 115, the Q node 104, and the Q′ node 110. An nFET device(M₅) 111 is connected to a second bit line node (BLB) 118, the WL node116, and the Q′ node 110. The nFET devices, M₅ and M₆, respectively, arethe pass-gate (PG) transistors of the SRAM; the nFET devices, M₁ and M₃,respectively, are the pull-down (PD) transistors of the SRAM; and thepFET devices, M₂ and M₄, respectively, are the pull-up (PU) transistorsof the SRAM.

In the present invention, nanowires which are used to provide FETs forinverters and SRAM cells are oriented perpendicularly to the surface ofthe silicon wafers on which they are formed. Reference is now made toFIG. 2, which is a plan view taken in an axial direction of thenanowires from a vantage point perpendicularly displaced from thesilicon wafer 200 on which the nanowires are formed and illustrating anarrangement of such nanowires useful in the practice of the presentinvention. As such, the nanowires are shown as circles since they arebeing viewed end on. It will be noted that the nanowires are labelled M₁through M₆, so that reference may be made to the circuit diagram in FIG.1 from time to time in the discussion to follow. So, nanowires 201 to206 are used in the manufacture of FETs M₁ through M₆, respectively, andare doped as necessary to produce pFETs (M₂, M₄) and nFETs (M₁, M₂, M₅,M₆). It should be understood that the nanowires 201 to 206 are graduallybuilt up to their intended height (length) upward from the silicon wafer200 in the course of this manufacture.

The FETs are assembled in layers on the silicon wafer 200. Thebottommost layer (layer 1) is shown in FIG. 3, taken, as are allsubsequent views, from the same vantage point as was FIG. 2. FIG. 3illustrates the bottom contacts for nanowires 201 to 206. It will benoted that nanowires 201, 203 for nFETs M₁, M₃, respectively, share abottom contact 301 with Vss as suggested in the circuit diagram inFIG. 1. Further, nanowires 202, 204 for pFETs M₂, M₄, respectively, havea bottom contacts 302, 303, respectively, with Vdd as suggested in thecircuit diagram in FIG. 1. Finally, nanowire 205 for nFET M₅ has abottom contact 304 with the second bit line node (BLB), and nanowire 206for nFET M₆ has a bottom contact 305 with the first bit line node (BL),both as suggested in the circuit diagram in FIG. 1. It should beunderstood that, while nanowires 201 to 206 are shown by themselves inFIG. 2 for purposes of illustration, they may be disposed upstanding ontheir respective bottom contacts or set within and projecting upwardfrom them so that their bottom ends are surrounded by the materialthereof.

The next layer (layer 2) above the bottommost layer is shown in solidlines in FIG. 4, which illustrates the gate patterning for FETs M₁ toM₆. Layer 2, it is to be understood, is vertically separated, or, morespecifically, separated in a direction perpendicular to the siliconwafer 200, from the bottommost layer (layer 1). Gate contact 401surrounds nanowires 201, 202 for nFET M₁ and pFET M₂, respectively, assuggested by the circuit diagram in FIG. 1, where the gates for M₁ andM₂ are connected to one another. Gate contact 401 has a gate extension402, whose purpose will be given below. Similarly, gate contact 403surrounds nanowires 203, 204 for nFET M₃ and pFET M₄, respectively, assuggested by the circuit diagram in FIG. 1, where the gates for M₃ andM₄ are connected to one another. Gate contact 403 has a gate extension404, whose purpose will also be given below. Gate contacts 405, 406surround nanowires 205, 206, respectively, for nFETs M₅ and M₆, bothultimately to be attached to the word line (WL), as suggested by thecircuit diagram in FIG. 1.

The next layer (layer 3) above the gate patterning layer (layer 2) isshown in solid lines in FIG. 5, which illustrates the top contacts fornanowires 201 to 206. Layer 3, it is to be understood, is verticallyseparated, or, more specifically, separated in a direction perpendicularto the silicon wafer 200, from the gate patterning layer (layer 2). Itwill be noted that nanowires 201, 202, 205 for nFET M₁, pFET M₂, andnFET M₅, respectively, share a top contact 501; nFET M₁, pFET M₂, andnFET M₅ are all connected to one another as shown in the circuit diagramin FIG. 1. Further, nanowires 203, 204, 206 for nFET M₃, pFET M₄, andnFET M₆, respectively, share a top contact 502; nFET M₃, pFET M₄, andnFET M₆ are likewise all connected to one another as shown in thecircuit diagram in FIG. 1. Although nanowires 201 to 206 are visible inFIG. 5 for purposes of illustration, the top contacts 501, 502 covertheir respective top ends.

It will be recalled that nFETs M₁ and M₃ are referred to as pull-down(PD) transistors; that nFETs M₅ and M₆ are referred to as pass-gate (PG)transistors; and that pFETs M₂ and M₄ are referred to as pull-up (PU)transistors. The partially assembled SRAM cell shown in FIG. 5 may bedescribed as having a pull-up/pass-gate/pull-down (PU-PG-PD) ratio of1-1-1, meaning that each of the transistors is being constructed usingone nanowire. Cells having other ratios may be constructed, and the useof different numbers of nanowires to form the transistors provides agood way to ratio the transistors making up a SRAM cell.

For example, FIG. 6 shows a partially assembled SRAM cell having aPU-PG-PD ratio of 1-1-2 in which pull-down (PD) transistors M₁ and M₃are both constructed using two nanowires. Specifically, nFET M₁ has asecond nanowire 207, and nFET M₃ has a second nanowire 208. It isnecessary, in order to provide the necessary bottom contacts for theadditional nanowires 207, 208, that bottom contact 601 be extended ateach end, namely, to the left for nanowire 207 and to the right fornanowire 208, compared to bottom contact 301 of FIG. 3, to contactnanowires 207, 208 in addition to nanowires 201, 203.

Similarly, gate contacts 602, 603 are extended, compared to gatecontacts 401, 403 of FIG. 4, to surround nanowires 207, 208 in additionto nanowires 201, 203, respectively. Finally, top contacts 604, 605 areextended, compared to top 501, 502 of FIG. 5, to contact nanowires 207,208 in addition to nanowires 201, 203.

As an additional example, FIG. 7 shows a partially assembled SRAM cellhaving a PU-PG-PD ratio of 1-2-2 in which pull-down (PD) transistors M₁and M₃ are both constructed from two nanowires, and pass-gate (PG)transistors M₅ and M₆ are both constructed from two nanowires. Inaddition to the details provided above for FIG. 6, nFET M₅ has a secondnanowire 209, and nFET M₆ has a second nanowire 210. In order to providethe necessary bottom contacts for the additional nanowires 209, 210,bottom contact 701 is extended, compared to bottom contact 304 of FIG.3, to the left to contact nanowire 209 in addition to nanowire 205, andbottom contact 702 is extended to the right, compared to bottom contact305 of FIG. 3, to contact nanowire 210 in addition to nanowire 206.

Similarly, gate contacts 703, 704 are extended, compared to gatestructures 405, 406 of FIG. 4, to surround nanowires 209, 210 inaddition to nanowires 205, 206, respectively. Finally, top contacts 705,706 are extended, compared to top contacts 604, 605 of FIG. 6, tocontact nanowires 209, 210 in addition to nanowires 201, 202, 205, 207and 203, 204, 206, 208, respectively.

Returning now to the embodiment having a PU-PG-PD ratio of 1-1-1 shownin FIG. 5, the next layer (layer 4) to be formed above the top contacts501, 502 is a first metallization layer. However, it is first necessaryto provide vias for connecting bottom contacts 301, 302, 303, 304, 305;gate contacts 401, 403, 405, 406; and top contacts 501, 502 to the firstmetallization layer. The vias connecting these contacts to the firstmetallization layer are collectively referred to as Via0. Referring toFIG. 8, bottom vias 801, 802, 803, 804, 805 are provided on source-sidecontacts 301, 302, 303, 304, 305, respectively, and extend more or lessperpendicularly therefrom in a direction parallel to the nanowires 201,202, 203, 204, 205, 206.

Similarly, gate vias 806, 807, 808, 809 are provided on gate contact405, gate extensions 402, 404, and gate contact 406, respectively, andextend more or less perpendicularly therefrom in a direction parallel tothe nanowires 201, 202, 203, 204, 205, 206. Finally, top vias 810, 811are provided on top contacts 501, 502, respectively, and also extendmore or less perpendicularly therefrom in a direction parallel to thenanowires 201, 202, 203, 204, 205, 206.

Bottom vias 801, 802, 803, 804, 805; gate vias 806, 807, 808, 809; andtop vias 810, 811, collectively Via0, extend above the top contact layer(layer 3) shown in FIG. 5 to a first metallization layer (layer 4) in aplane separated from top contacts 501, 502. In other words, layer 4 isvertically separated, or, more specifically, separated in a directionperpendicular to the silicon wafer 200, from the top contact layer(layer 3).

Reference is now made to FIG. 9, where the first metallization layer(layer 4) is shown to include strips of metallic conductor depositedonto the structure shown in FIG. 8 to form some of the necessaryelectrical connections for a cell. Strips 901, 902, providingconnections to a word line (WL) node, make electrical contact with gatevias 806, 809, respectively, for nFETs M₅ and M₆, in accordance with thecircuit diagram in FIG. 1. Strips 903, 904, providing connections to avoltage source node (Vdd), make electrical contact with bottom vias 802,803, respectively, for pFETs M₂ and M₄, in accordance with the circuitdiagram in FIG. 1. Strips 901 to 904 may continue to the left and to theright to provide the same connections for SRAM cells to the left andright of that being illustrated in FIG. 9.

The first metallization layer (layer 4) also includes strips 905, 906,907 of metallic conductor, which make electrical contact with bottomvias 801, 804, 805, respectively. These will ultimately provideconnections to a ground node (Vss), to a first bit line node (BL), andto a second bit line node (BLB), respectively, using second and thirdmetallization layers to be described below. In this regard, ground node(Vss) will ultimately be connected to the bottom contact 301 for nFETsM₁ and M₃; first bit line node (BL) will ultimately be connected to thebottom contact 305 for nFET M₆; and second bit line node (BLB) willultimately be connected to the bottom contact 304 for nFET M₅, all inaccordance with the circuit diagram in FIG. 1.

The first metallization layer (layer 4) finally includes L-shaped strips908, 909 of metallic conductor. L-shaped strip 908 connects gate via 808to top via 810, whereby the gate contact 403 for nFET M₃ and pFET M₄ areconnected to the top contact 501 for nFET M₁, pFET M₂, and nFET M₅, inaccordance with the circuit diagram in FIG. 1. Similarly, L-shaped strip909 connects gate via 807 to top via 811, whereby the gate contact 401for nFET M₁ and pFET M₂ are connected to the top contact 502 for nFETM₃, pFET M₄, and nFET M₆.

Still referring to FIG. 9, subsequent to the deposition of strips ofmetallic conductor 901 to 909, vias, collectively referred to as Via1,extend above the first metallization layer (layer 4) from strips 905,906, 907 of metallic conductor to a second metallization layer (layer 5)in a plane separated from strips 901 to 909. In other words, the secondmetallization layer (layer 5) is vertically separated, or, morespecifically, separated in a direction perpendicular to the siliconwafer 200, from the first metallization layer (layer 4).

Turning now to FIG. 10, vias 1001, 1002, 1003, collectively Via1, extendupward from strips 905, 906, 907 of metallic conductor to a secondmetallization layer (layer 5) in a plane separated from strips 901 to909. The second metallization layer (layer 5) comprises strips 1004,1005 of metallic conductor. Strips 1004, 1005, providing connections toa first bit line node (BL) and a second bit line node (BLB),respectively, make electrical contact with vias 1002, 1003,respectively, which, in turn, make electrical contact with bottom vias804, 805 for nFETs M₆ and M₅, in accordance with the circuit diagram inFIG. 1.

Still referring to FIG. 10, subsequent to the deposition of strips 1004,1005 of metallic conductor, a via, which may also be referred to asVia2, extends above the second metallization layer (layer 5) from via1001 to a third metallization layer (layer 6) in a plane separated fromstrips 1004, 1005. In other words, the third metallization layer (layer6) is vertically separated, or, more specifically, separated in adirection perpendicular to the silicon wafer 200, from the secondmetallization layer (layer 5).

Turning now to FIG. 11, via 1101, otherwise known as Via2, extendsupward from via 1001 to a third metallization layer (layer 6) in a planeseparated from strips 1004, 1005. The third metallization layer (layer6) comprises strip 1102 of metallic conductor. Strip 1102, providing aconnection to a ground node (Vss), makes electrical contact with via1001, which, in turn, makes electrical contact with bottom via 801 fornFETs M₁ and M₃, in accordance with the circuit diagram in FIG. 1.

This completes the description of a single SRAM cell 1010 assembled inaccordance with the present invention. It should be observed that theheight of the SRAM cell 1110, the height being in the vertical directionin FIG. 11, is eight times the distances separating the nanowires fromone another, the latter being referred to as the nanowire pitch. Thewidth of the SRAM cell 1110, the width being in the horizontal directionin FIG. 11, is three times the metal pitch, the latter being thedistance separating adjacent strips of metallic conductor in the secondmetallization layer (layer 5) to be more clearly shown in FIG. 12. Whenthe nanowire pitch is 18 nanometers (nm), and the metal pitch is 24 nm,the area of a single SRAM cell 1110 is 0.0104 μm².

Referring back to FIGS. 6 and 7, which show partially assembled SRAMcells having PU-PG-PD ratios of 1-1-2 and 1-2-2, respectively, it willbe recognized that the areas of SRAM cells of those types will be thesame as that of the 1-1-1 SRAM cell 1110 shown in FIG. 11. That isbecause the additional nanowires required for the production of suchcells will not extend beyond the area of the SRAM cell 1110 as shown, asthey will be located below (underneath) the strips 1004, 1005 ofmetallic conductor.

Turning now to FIG. 12, the location of SRAM cell 1110 relative to itsneighbors in the vertical and horizontal directions is shown. It will bereadily apparent to those of ordinary skill in the art that the SRAMcell of the present type may be disposed on a silicon wafer in apattern, such as that shown in FIG. 12, having very little unused area.It will also be noted that strips 901, 902, 903, 904, 1104, 1105, 1102of metallic conductor continue from SRAM cell 1110 to make the necessaryconnections in neighboring SRAM cells in the vertical and horizontaldirections.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an”, and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising”, when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

Various modifications and adaptations may become apparent to thoseskilled in the relevant arts in view of the foregoing description, whenread in conjunction with the accompanying drawings. However, any and allmodifications of the teachings of this disclosure will still fall withinthe scope of the non-limiting embodiments of this invention.

Although described in the context of particular embodiments, it will beapparent to those skilled in the art that a number of modifications andvarious changes to these teachings may occur. Thus, while the inventionhas been particularly shown and described with respect to one or moreembodiments thereof, it will be understood by those skilled in the artthat certain modifications or changes may be made therein withoutdeparting from the scope of the invention as set forth above, or fromthe scope of the claims to follow.

1.-12. (canceled)
 13. A method for forming a memory device, said methodcomprising: forming a first plurality of semiconductor nanowires for afirst, a second, and a third field effect transistor (FET) and a secondplurality of semiconductor nanowires for a fourth, a fifth, and a sixthFET on a substrate, said first and second pluralities of semiconductornanowires being arranged with said semiconductor nanowires beingoriented substantially perpendicular to said substrate; forming aplurality of bottom contacts for said semiconductor nanowires on saidsubstrate, each of said semiconductor nanowires having a bottom contact,the one or more of said semiconductor nanowires of said first pluralityfor said third FET sharing a bottom contact with the one or more of saidsemiconductor nanowires of said second plurality for said fourth FET;forming a plurality of gate contacts separated in a directionperpendicular to said substrate from said plurality of bottom contacts,each of said semiconductor nanowires of said first and secondpluralities having one of said gate contacts, two or more semiconductornanowires of said first plurality for said second and third FETs sharinga gate contact with one another, and two or more semiconductor nanowiresof said second plurality for said fourth and fifth FETs sharing a gatecontact with one another; forming a first top contact for said firstplurality of semiconductor nanowires and a second top contact for saidsecond plurality of semiconductor nanowires, said first and second topcontacts separated in a direction perpendicular to said substrate fromsaid plurality of gate contacts, whereby said third and fourth FETsshare a bottom contact; said second and third FETs share a gate contact;said fourth and fifth FETs share a gate contact; said first, second, andthird FETs share a top contact; and said fourth, fifth, and sixth FETsshare a top contact; connecting said gate contact of said second andthird FETs to said top contact of said fourth, fifth, and sixth FETs;connecting said gate contact of said fourth and fifth FETs to said topcontact of said first, second, and third FETs; connecting said gatecontacts of said first and sixth FETs to word line (WL) connections;connecting said bottom contacts of said second and fifth FETs to voltagesource (Vdd) connections; connecting said bottom contact of said firstFET to a second bit line (BLB) connection; connecting said bottomcontact of said sixth FET to a first bit line (BL) connection; andconnecting said shared bottom contact of said third and fourth FETs to aground (Vss) connection.
 14. The method as claimed in claim 13, whereinsaid first and second pluralities of semiconductor nanowires are siliconnanowires.
 15. The method as claimed in claim 13, wherein said first FETis an n-type FET (nFET), said second FET is a p-type FET (pFET), saidthird FET is an nFET, said fourth FET is an nFET, said fifth FET is apFET, and said sixth FET is an nFET.
 16. The method as claimed in claim13, wherein said first FET is a pass-gate (PG) transistor, said secondFET is a pull-up (PU) transistor, said third FET is a pull-down (PD)transistor, said fourth FET is a PD transistor, said fifth FET is a PUtransistor, and said sixth FET is a PG transistor.
 17. The method asclaimed in claim 13, wherein said first, second, fifth, and sixth FETsare each formed by one semiconductor nanowire, and said third and fourthFETs are each formed by two semiconductor nanowires.
 18. The method asclaimed in claim 13, wherein said second and fifth FETs are each formedby one semiconductor nanowire, and said first, third, fourth, and sixthFETs are each formed by two semiconductor nanowires.
 19. The method asclaimed in claim 13, wherein said first, second, third, fourth, fifth,and sixth FETs are each formed by one semiconductor nanowire.
 20. Themethod as claimed in claim 13, wherein said gate contact of said secondand third FETs is connected to said top contact of said fourth, fifth,and sixth FETs in a first metallization layer separated in a directionperpendicular to said substrate from said first and second top contacts;said gate contact of said fourth and fifth FETs is connected to said topcontact of said first, second, and third FETs in said firstmetallization layer; said gate contacts of said first and sixth FETs areconnected to word line (WL) connections in said first metallizationlayer by vias extending from said gate contacts in a directionsubstantially perpendicular to said substrate to said word line (WL)connections; said bottom contacts of said second and fifth FETs areconnected to voltage source (Vdd) connections in said firstmetallization layer by vias extending from said bottom contacts in adirection substantially perpendicular to said substrate to said voltagesource (Vdd) connections; said bottom contact of said first FET isconnected to a second bit line (BLB) connection in a secondmetallization layer separated in a direction perpendicular to saidsubstrate from said first metallization layer by a via extending fromsaid bottom contact in a direction substantially perpendicular to saidsubstrate to said second bit line (BLB) connection; said bottom contactof said sixth FET is connected to a first bit line (BL) connection insaid second metallization layer by a via extending from said bottomcontact in a direction substantially perpendicular to said substrate tosaid first bit line (BL) connection; and said shared bottom contact ofsaid third and fourth FETs is connected to a ground (Vss) connection ina third metallization layer separated in a direction perpendicular tosaid substrate from said second metallization layer by a via extendingfrom said shared bottom contact in a direction substantiallyperpendicular to said substrate to said ground (Vss) connection.
 21. Themethod as claimed in claim 13, wherein said first FET is formed by morethan one of said first plurality of semiconductor nanowires.
 22. Themethod as claimed in claim 13, wherein said third FET is formed by morethan one of said first plurality of semiconductor nanowires.
 23. Themethod as claimed in claim 13, wherein said fourth FET is formed by morethan one of said second plurality of semiconductor nanowires
 24. Themethod as claimed in claim 13, wherein said sixth FET is formed by morethan one of said second plurality of semiconductor nanowires.